The present invention relates to systems and methods for treating process material and, more particularly, to systems and methods for treating municipal solid waste material, medical waste material, reclaimed paper and the like.
As a result of increasing scarcity of landfills and more stringent environmental regulations, efforts have been made to reduce the volume of process material, such as municipal solid waste (xe2x80x9cMSWxe2x80x9d) and paper material, such as newsprint and other reclaimed and recycled paper products as a step in the process of disposing of the material, either by depositing it in landfills, incinerating it or recycling it. Processes have been developed to break down such material for disposal, or in the case of paper products, use as insulation. An example of such a process and device is Holloway U.S. Pat. No. 5,190,226. That patent discloses an apparatus and method for separation, recovery and recycling of MSW. The apparatus includes a rotating drum which is fed at an upstream end by a reciprocating ram, a steam source which is connected to introduce high temperature steam into the drum, and a spiral rib or flight mounted within the drum to transport material deposited in an upstream end of the drum along the length of the drum.
The drum is maintained in a pressurized state during a sterilization process by inlet and exit air locks which are connected to and enclose the inlet and outlet of the rotating drum. The outlet air lock is connected to a rotating trommel which receives sterilized waste from the drum and rotates it over screens to provide a sizing function.
In view of heightened concerns over the spread of infectious diseases, many of which are fatal to humans, governmental agencies have imposed strict requirements on the disposal of waste, and more particularly, medical waste. Such medical waste may include relatively soft items such as hospital linen, garments worn by patients and health care providers, bandages, gauze and other fabric material which comes in contact with a patient, as well as harder materials such as disposable plastic surgical instruments and hypodermic needles. In order to treat such materials to render them safe and at the same time comply with federal, state and local standards, it is often necessary to grind, shred, or otherwise comminute such medical waste, and in addition, to sterilize the waste by heating it, contacting it with a disinfectant, or a combination of both. Furthermore, as in most sterilization processes, it is necessary that the actual sterilization steps be conducted in a closed and controlled atmosphere.
An example of such a device and method is shown in Lewis et al. U.S. Pat. No. 5,941,468. That patent discloses a medical waste sterilization system in which medical waste is shredded and placed in a rotating autoclave cylinder where it is first tumbled and contacted by steam to heat it, then is heated further in a downstream section of the autoclave cylinder which is surrounded by a steam jacket. The waste is then cooled and dehydrated in a third section of the cylinder and deposited in a compactor. The object of the device and method is to heat the medical waste thoroughly to a sterilizing temperature (typically above 212xc2x0 F. for at least 30 minutes), then dehydrate the material to reduce its volume and weight.
A disadvantage with such systems is that, while they may be effective in treating MSW or process material, or sterilizing medical waste, they either operate on a batch process, that is, as in the device of aforementioned U.S. Pat. No. 5,190,226 the heating vessel is first loaded with solid waste, rotated to expose the waste to steam at elevated temperature and pressure, then the waste is evacuated from the rotating drum or pressure vessel completely before new waste is introduced for sterilization; or, as in the device of U.S. Pat. No. 5,941,468, the vessel is maintained below ambient pressure since no air-tight seal is effected in the waste material inlet and outlet openings of the vessel.
A second disadvantage with such systems is that the heating of material is performed entirely within a rotating drum or other vessel which typically has a volumexe2x80x94much larger than that of the waste material to be sterilized. As a result, the entire interior volume of the drum must be heated to the desired temperature, and the waste tumbled to expose it to steam so that it is heated evenly. Consequently, a larger amount of steam, and therefore heat energy, is required to heat the entire interior of the vessel, in comparison to the amount of steam and heat energy to required to heat simply the waste material itself.
Accordingly, there is a need for a material treatment system and method which can utilize a pressurized vessel operated on a continuous basis, as opposed to being operated on a batch basis, the latter process having the disadvantage of providing a reduced throughput rate, and further, requires successive heating and cooling cycles which require relatively large amounts of energy. There is also a need for a system and method for treating material, including medical waste, in which the energy needed to heat the material is concentrated on the material itself, as opposed to a process in which material is heated simply by placing it in a large volume, the entirety of which must be heated to the necessary temperature.
The present invention is a system and method for treating material under pressure in which the material is compacted into a plug within an inlet extrusion tube and preheated, then heated to the desired temperature within a rotating autoclave vessel fed by the extrusion tube. In a preferred embodiment of the present invention, the system includes inlet and outlet extrusion tubes through which material to be treated enters and exits a pressurized treatment zone which includes the autoclave vessel. Inlet and outlet injector screws feed material into the inlet and outlet extrusion tubes, respectively, to create continuously moving or xe2x80x9cdynamic,xe2x80x9d air-tight plugs of material within the tubes to seal the treatment zone.
Since it is the material itself that forms the seals of the pressurized treatment zone inlet and outlet, the system of the present invention can perform a treating function on a continuous basis, even though the vessel is pressurized above ambient. This design represents an improvement over prior art devices in which material treatment is performed in autoclaves which are sealed with a conventional gate or door, such that the treatment process must be performed in a batch mode. As a result, the system and process of the present invention provides a greater throughput rate than prior art designs and processes, and a concomitant greater energy efficiency, since the pressurized treatment zone does not need to be successively heated to treat material, then cooled to allow the material to be removed at a safe handling temperature.
The system and method of the present invention is effective in treating medical waste, in which case the material is heated and retained within the pressurized treatment zone sufficiently to sterilize it, preferably being heated to at least 212xc2x0 F., as well as MSW and paper, such as newsprint, in which case the material is heated and retained within the pressurized treatment zone sufficiently to xe2x80x9ccookxe2x80x9d it and break it down, especially the paper fibers. Medical waste, MSW and paper shall be referred to herein collectively as xe2x80x9cmaterial.xe2x80x9d
In a preferred embodiment of the invention, the system has an input component which includes a conveyor which conveys material through an air lock to a hopper, a shear shredder which receives material from the hopper and shreds it to reduce its size, and an injector screw which receives the shredded material and feeds it to the inlet extrusion tube of the pressurized treatment zone. In alternate embodiments, the input component includes a hopper having a reciprocating ram which forces material into the shear shredder, and an air lock, located between the shear shredder and the injector screw, which eliminates the need for an air lock associated with the conveyor.
The central component of the present invention is the pressurized treatment zone, which includes a rotating autoclave vessel. Essential to the operation of the rotating autoclave vessel is the heated inlet extrusion tube which connects the inlet feed screw with the interior of the autoclave vessel. The inlet extrusion tube preferably comprises a cylindrical tube having a slightly diverging interior wall, a chamber for receiving pressurized, heated steam, and an inner face with orifices shaped and oriented to direct steam from the interior of the extrusion tube into the interior of the autoclave. It is within this extrusion tube that material received by the inlet injector screw and compacted to form an air-tight plug, heated and largely sterilized (if desired) prior to its entering the autoclave vessel. Since the material is compacted and in intimate contact with the heated extrusion tube, the heating is more efficient than in prior art vessels which are largely empty during operation.
The autoclave vessel is a horizontally-oriented cylinder which is mounted for rotation. The interior of the vessel includes a continuous, helical rib so that rotation of the vessel about its central, longitudinal axis causes material deposited within it to progress along the length of the vessel. The interior of the vessel is heated exclusively by the steam which exits the orifices formed in the extrusion tube. The purpose of the vessel is to fluff the material and expose it thoroughly to the heat from the steam, which will sterilize medical waste and cook and break down MSW and paper pulp. The helical rib located within the vessel includes a plurality of longitudinally-extending ribs which extend between flights and act to lift the hazardous material and tumble it as it progresses along the vessel. This further ensures the even heating and sterilization of the material.
In the preferred embodiment, the vessel includes an open exit end which empties into a stationary, vertically-oriented pressure vessel or hopper. That pressurized hopper directs the now-treated material downwardly into a second or exit injector screw which feeds it into a second or exit extrusion tube, forming a plug of material. This downstream or exit plug of material also acts as a seal so that the atmosphere within the pressure vessel, pressurized hopper, and exit injector screw is sealed from the environment. The exit extrusion tube preferably is connected to, and therefore deposits treated material into, a tumbler, where the material is dehydrated and cooks. The tumbler preferably includes an exhaust blower which maintains the interior of the tumbler at a slightly-below-atmospheric pressure. The tumbler also includes a helical rib which causes the material to break apart as it progresses along the tumbler, and the tumbler is open at its exit end so that the material may be deposited upon a conveyor for removal.
In an alternate embodiment, the exit injector screw and exit extrusion tube are replaced by an air lock which acts to seal the interior of the vessel and stationary exit vessel from the ambient. Also in alternate embodiments, the exit air lock is replaced with a sealable door so that the sterilization process may be conducted, if desired, on a batch basis. With that embodiment, the pressure vessel processes material along its length, then after the material has been adequately heated, the exit door is opened and the treated material is forced outwardly through it by the helical ribs within the vessel unto a conveyor for removal. However, the inlet injector screw and inlet extrusion tube of that embodiment allow continuous feeding into the pressurized autoclave vessel during treatment.
With all of the embodiments summarized above, the input component, which includes an injector screw, creates a plug within an inlet extrusion tube which acts as a dynamic seal, since the material progressing through the extrusion tube into the vessel seals the interior of the autoclave vessel from the ambient and from the shredding and input segments of that component.
The method of the present invention includes the steps of receiving material within an enclosed environment, shredding the material within that environment, forming a dynamic plug of shredded material while simultaneously heating the material to a predetermined temperature (preferably greater than 212xc2x0 F.), depositing the heated material into a rotating vessel where it is broken up and maintained at a temperature above 212xc2x0 F., then cooling and dehydrating the material. Also in the preferred method, the treated material is formed into a dynamic plug which seals the pressurized treatment zone of the apparatus.
Accordingly, it is an object of the present invention to provide a system and apparatus for treating material in which the material can be heated under pressure on a continuous basis if desired; a system and method for treating waste in which the waste itself is formed into dynamic plugs which form seals to isolate the heating regions of the apparatus from the input and exit regions; and a system and method for heating material which is rugged and can be modified easily to perform treatment of material on a batch basis, if desired.